Pachnodus snail (Pachnodus velutinus)

Kingdom

Animalia

Phylum

Mollusca

Class

Gastropoda

Order

Stylommatophora (2)

Family

Cerastuidae

Genus

Pachnodus (1)

Classified as Extinct (EX) on the IUCN Red List (2).

The now extinct Pachnodus velutinus was one of a number of land-snail species endemic to the Seychelles islands. A diminutive species, the shell of Pachnodus velutinus was less than 1.5 centimetres wide, with five whorls, and coloured a matt, uniform beige, while, in contrast, the body was black (1). Although the shell was always very thin and poorly calcified, some specimens lacked a calcified shell layer altogether, possessing only a thin outer layer, the periostracum, made of hardened protein (3).

Pachnodus velutinus was endemic to the island of Mahé in the Seychelles. Surveys in 1972 indicated that it occupied a limited range of around two square kilometres within Morne Seychellois National Park in the north of island. However, over the period 1972 to 1994, this species’ range rapidly contracted. By 1987, Pachnodus velutinus was only found in the forests of the Congo Rouge Mountain, and by 1994, it had disappeared altogether (3).

Although little is known about the biology of Pachnodus velutinus, one interesting and important feature was its inability to undergo aestivation (3), a period of dormancy that occurs when conditions are hot and dry, analogous to hibernation in cold environments (5). Most other species of the genus Pachnodus are capable of forming a mucous seal between the aperture of their shell and the substrate they are resting upon, thereby preventing moisture loss and allowing the snails to survive when humidity levels drop below 70 percent. However, Pachnodus velutinus lacked this ability, and could only survive at humidities of over 80 percent, below which it would quickly dry up and die (3).

As a result of breeding between Pachnodus velutinus and the closely related snail, Pachnodus niger, a hybrid species was created, Pachnodus niger x velutinus. This species, still in existence, has features of both parent species, but, most importantly, it has the strong, well-calcified shell of Pachnodus niger, allowing it to undergo aestivation. Both Pachnodus velutinus and the hybrid co-existed on Mahé as regionally separate populations until the 1960s, when clearance of mid-altitude forest (3), and possibly global climate change (6), may have caused the local climate within the ranges of both species to become significantly less humid (3) (6). Since it was able to aestivate, the hybrid species was better adapted to survive in the mid-altitude regions than Pachnodus velutinus and, as a result, from 1972 to 1987 it rapidly expanded its range. As the hybrid encroached on the range of Pachnodus velutinus the two snail species bred together, producing hybrid offspring, and thereby causing the range of pure Pachnodus velutinus individuals to contract. By 1987, the only surviving pure populations of Pachnodus velutinus were in the dampest, most climatically stable part of its former range, the high-altitude Congo Rouge mountain region. At this point the range of Pachnodus velutinus had become so small that random mating with the hybrid snails gradually bred this species out of existence (3).

The extinction of Pachnodus velutinus may represent an important example of the diverse impacts that can result from climate change in local ecosystems. Despite reforestation of degraded areas in Morne Seychellois National Park (4), the effects of the change in forest composition (3), and perhaps global climate change (6), have already contributed to the loss of a unique component of Seychelles’ biodiversity. This is the first example of hybrid superiority causing the extinction of a parent species, and is also one of the few examples where an extinction process has been followed all the way to its conclusion (3). Hopefully, the study of this event will provide useful information for the prevention of future extinctions.